Ever noticed how when you rub your dog’s belly, they kick their leg out? It’s actually a reflex, like the one that human’s have in their knee.
Animal Planet describes the strange action. “Dogs shake or kick their legs when you scratch them because of something known as the scratch reflex. It’s a completely involuntary reaction, which explains why your dog may look as puzzled as you do when it starts to happen. When you scratch or tickle your dog’s belly, it irritates him, much the same way that the wind or a bug might. It activates nerves under his skin that are connected to his spinal cord and relays a message to his leg muscles to kick in an attempt to get rid of the irritant. Of course, you’re the one both providing the “itch” and “scratching” it, so it’s a completely pointless act.”
But this unusual and ‘pointless’ behaviour isn’t entirely useless. Vet’s can use this scratch reflex to assist in diagnosing nerve damage or neurological problems.
What’s really strange is that even though it’s known as an irritant and the action happens in order to stop the irritation, the dog actually likes the scratching. So find a different spot that doesn’t cause their leg to kick and they’ll enjoy it much more.